1. Field of the Invention
The present invention relates to a power supply apparatus, and more particularly to a power supply apparatus with a current-sharing function.
2. Description of Prior Art
In electronic engineering, a DC-to-DC converter is an electronic circuit which converts a source of direct current (DC) from one voltage level to another, and the converted DC voltage is stabilized at the preset voltage value. Generally speaking, the DC-to-DC converter is divided into two categories: one is “step-down” DC-to-DC converter (namely, the output voltage is lower than the input voltage), and the other one is “step-up” DC-to-DC converter (namely, the output voltage is higher than the input voltage). The DC-to-DC converter is mainly applied to a distributed power system. Hence, the DC voltage level of the previous stage is fixed, and the DC voltage level of the next stage can be connected to the corresponding DC-to-DC converter according to the required power.
More particularly, the DC-to-DC converter can be separated into two categories: the pulse width modulation (PWM) converter and the resonant converter. The hard-switching operation of the PWM converter introduces the high switching losses and the poor efficiency. Accordingly, the soft-switching technologies have been developed for the resonant converter to reduce the switching losses and increase the efficiency.
The DC characteristic of the resonant converter could be divided into ZVS (zero-voltage switching) region and ZCS (zero-current switching) region. Accordingly, the resonant circuit structure is adopted in high-efficiency and high-power power circuits. Reference is made to FIG. 1 which is a circuit diagram of a prior art LLC resonant circuit. The LLC resonant circuit includes a DC voltage 100, a square-wave generating circuit 102, a resonant circuit 104, a conversion circuit 106, and a rectifier-filter circuit 108.
The square-wave generating circuit 102 is composed of two semiconductor components QT, QB, and on-state and off-state of the two semiconductor components QT,QB are controlled by a controller (not labeled). Hence, the square-wave generating circuit 102 can generate two different voltage levels. The resonant circuit 104 is composed of a resonant capacitor Cr and two primary windings of two transformers T1,T2. The resonant capacitor Cr is provided to filter a DC component of a pulsating voltage generated by the square-wave generating circuit 102. Also, each of the primary windings is provided to transform electrical energy into magnetic energy, and the transformed magnetic energy is delivered to corresponding secondary windings of the transformers T1, T2. The turns of the secondary windings can be represented as follows:Nsecondary—as1+Nsecondary—as2=2*Nprimary*Vout/Vin=2*Nsecondary
Where, the terms Nsecondary_as1 and Nsecondary_as2 are the turns of the secondary windings, the term Nprimary is the turns of the primary winding, the term Vin is the input voltage of the primary winding, the term Vout is the output voltage of the secondary winding, and the term Nsecondary is calculated turns of the secondary windings.
The rectifier-filter circuit 108 is composed of four diodes D1, D2, D3, D4 and a filter capacitor Co. The function of rectifying and filtering is implemented based on the single-directional turn-on property of the diodes D1, D2, D3, D4 and the charging and discharging property of the filter capacitor Co. The description of operating the LLC resonant converter is as follows. First, a pulsating voltage is generated at the point A when the DC voltage 100 inputs to the square-wave generating circuit 102. Afterward, the resonant capacitor Cr filters the DC component of the pulsating voltage and the AC component of the pulsating voltage is resonated when the pulsating voltage passes through the resonant circuit 104. Afterward, the AC voltage and the AC current are outputted at the conversion circuit 106, namely the secondary windings of the transformers T1, T2. Finally, the AC voltage is converted (rectified) into a DC voltage and the DC voltage is outputted from the rectifier-filter circuit 108.
References are made to FIG. 2(a) and FIG. 2(b), each of which is a diagram shows a B-H curve of a transformer Ta shown in FIG. 1 and a B-H curve of a transformer T2 shown in FIG. 1. It is clear to observe that the operation of the transformer core of the transformer T1 is in the first quadrant and the operation of the transformer core of the transformer T2 is in the third quadrant. Hence, the unbalanced operation of the transformer core is easily to saturate the transformer coils of the transformers T1, T2 and cause electrical shorts in the circuits. In addition, the amount of the rectifier diodes and the turns of the secondary windings are large, thus increasing the losses and reduce the efficiency.